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24 August 2011, 14:04 | Updated: 30 March 2016, 13:50
The locks at Wakefield Prison may have to be changed after an officer took his keys home.
An urgent inquiry was launched after the incident at HMP Wakefield - a top security jail which houses some of the UK's most notorious offenders.
A spokesman for the Prison Service said:
'A member of staff at HMP Wakefield inadvertently took their keys out of the prison for a short period.
'The incident was investigated as a matter of urgency, which included a forensic investigation of the key.
'Commenting on a relock of a prison is a potential breach of security.'
Breaches of security relating to keys are treated extremely seriously by prisons.
In June 2006 Feltham Young Offenders Institution had to be 'relocked' after an ITV documentary about an inquiry into the murder of an Asian teenager included a shot of a lock and key.
All locks in the west London establishment had to be changed at a cost of several hundred thousand pounds after the pictures were broadcast.
Experts said skilled locksmiths can make duplicate keys from detailed source material such as a close-up shot.
Wakefield Prison houses around 750 prisoners - mainly serious sex offenders including some with the highest possible security classification within the offender management system.
Among the notorious criminals who have been held at Wakefield are Soham killer Ian Huntley, crossbow cannibal Stephen Griffiths and Milly Dowler's murderer, Levi Bellfield.
Serial killer Harold Shipman killed himself in the jail in 2004.